The purpose of a catalytic converter for an internal combustion engine or a gas turbine is to convert pollutant materials in the exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides, etc., to carbon dioxide, nitrogen and water. Conventional catalytic converters utilize a ceramic honeycomb monolith having square or triangular, straight-through openings or cells with catalyst deposited on the walls of the cells; catalyst coated refractory metal oxide beads or pellets, e.g., alumina beads; or a corrugated thin metal foil monolith, e.g., a ferritic stainless steel foil or a nickel alloy foil, having a catalyst carried on or supported on the surface. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals, and or zeolite catalysts. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby the pollutant is converted to a harmless by-product which then passes through the exhaust system to the atmosphere.
However, conversion to such harmless by-products is not efficient initially when the exhaust gases are relatively cold. To be effective at a high conversion rate, the catalyst and the surface of the converter with which the gases come in contact must be at or above a minimum temperature e g., 390.degree. F. for carbon monoxide, 570.degree. F. for volatile organic compounds (VOC) and 1000.degree. F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. Once the exhaust system has reached its normal operating temperature, the catalytic converter is optimally effective. Hence, it is necessary for the relatively cold gases to make contact with a hot catalyst so as to effect satisfactory conversion. Compression ignited engines, spark ignited engines and reactors in gas turbines have this need.
To achieve initial heating of the catalyst prior to engine start-up, there is conveniently provided an electrically heatable catalytic converter, preferably one formed of a thin metal monolith, either flat thin metal strips, straight corrugated thin metal strips, pattern corrugated thin metal strips, (e.g., herringbone or chevron corrugated) or variable pitch corrugated thin metal strips (See U.S. Pat. No. 4,810,588 dated Mar. 7, 1989 to Bullock et al) or a combination thereof, which monolith is connected to a voltage source, e.g., a 12 volt to 108 volt, or higher, power supply, preferably at the time of engine start-up and afterwards to elevate and maintain the catalyst to at least 650.degree. F. plus or minus 20.degree. F. Alternatively, power may also be supplied for 2 to 10 or so seconds prior to start-up of the engine. Catalytic converters containing a corrugated thin metal (stainless steel) monolith have been known since at least the early seventies. See Kitzner U.S. Pat. Nos. 3,768,982 and 3,770,389 each dated Oct. 30, 1973. More recently, corrugated thin metal monoliths have been disclosed in U.S. Pats. No. 4,711,009 date Dec. 8, 1987; 4,381,590 to Nonnenmann et al dated May 3, 1983, and 5,070,694 dated Dec. 10, 1991 to William A. Whittenberger and entitled Electrically Heatable Catalytic Converter, commonly owned with the present application; and International PCT Publication Numbers WO 89/10470 and WO 89/10471 each filed Nov. 2, 1989. However, a common problem with any such prior devices has been their inability to survive severe automotive industry durability tests which are known as the Hot Shake Test and the Hot Cycling Test.
The Hot Shake Test involves oscillating (100 to 200 Hertz and 28 to 60 G inertial loading) the device in a vertical attitude at high temperature between 800.degree. and 950.degree. C.; 1472.degree. to 1742.degree. F., respectively) with exhaust gas from a running internal combustion engine simultaneously being passed through the device. If the electrically heatable catalytic device telescopes or displays separation or folding over of the leading, or upstream, edges of the foil leaves up to a predetermined time, e.g., 5 to 200 hours, the device is said to fail the test. Usually a device will fail within 5 hours if it is going to fail. Five hours is equivalent to 1.8 million cycles at 100 Hertz.
The Hot Cycling Test is conducted on a running engine at 800.degree. to 950.degree. C. (1472.degree. to 1742.degree. F.) and cycled to 120.degree. to 150.degree. C. once every 15 to 20 minutes, for 300 hours. Telescoping or separation of the leading edges of the foil strips is considered a failure.
The Hot Shake Test and the Hot Cycling Test, hereinafter called "Hot Tests", have proved very difficult to survive, and many efforts to provide a successful device have been either too costly or ineffective for a variety of reasons.
Previously tested samples of EHC's in automotive service and comprised entirely of heater strips in electrical parallel, did not have adequate endurance in Hot Tests or did they satisfy the need for lower power ratings. In repeated efforts to arrive at a suitable design sing purely parallel circuit construction, samples were made and tested with a wide range of parameters, including a length-to-diameter aspect ratio of from 0.5 to 1.5, cell densities of from 100 to 500 cells per square inch, individual strip heaters as long as 15 inches, and parallel circuits limited to as few as 2 heater strips.
Devices made according to these parameters proved unsatisfactory in the Hot Tests because (a) terminal resistance was too low and therefore, the device drew too much power, (b) a relatively high voltage differential between laminations associated with small numbers of parallel heater strips caused some arcing and, (c) Hot Tests could not be passed consistently. With regard to (c), EHC's with heater strips longer than about 7" measured from the center of the core to the outside of the core have generally not passed the Hot Shake Test. Resistance that is too low causes one or more of the following problems: (a) the weight and size of the battery become unacceptably high and/or expensive; and (b) the EHC has to be made so large in diameter that longer heater strips have to be used which induces a tendency to fail the Hot Tests. The devices were, of course, useful in less stringent environments.
Prior structures, such as that described in U.S. Pat. No. 4,928,485 dated May 29, 1990 to Whittenberger, have had all of the corrugated thin metal heater strip members connected in a manner such that all of the strips extended spirally outwardly from a central electrode to a circular shell which served as the electrode of opposite polarity. The strips served as heaters for the core. However, power levels of less than 2.0 kilowatts at 7 volts when measured at the EHC terminals could not be achieved when all of the heater strips were in parallel because the terminal resistance of the EHC was too low.
In the following description, reference will be made to "ferritic stainless steel." A suitable formulation for ferritic stainless steel alloy is described in U.S. Pat. No. 4,414,023 to Aggen dated Nov. 8, 1983. A specific ferritic stainless steel useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium and praseodymium, or a mixture of two or more of such rare earth metals, balance iron and trace steel making impurities. Another metal alloy especially useful herein is identified as Haynes 214 which is commercially available. This alloy is described in U.S. Pat. No. 4,671,931 dated Jun. 9, 1987 to Herchenroeder et al. Haynes 214 is characterized by high resistance to corrosion. A specific example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, trace amounts of one or more rare earth metals, 0.05% carbon, and steel making impurities. Ferritic stainless steel and Haynes 214 are examples of high temperature resistive, corrosion resistant metal alloys useful herein. Suitable alloys must be able to withstand temperatures of 900.degree. C. to 1100.degree. C. over prolonged periods.
Other high temperature resistive, corrosion resistant metal alloys are known and may be used herein. The thickness of the metal foil heater strips should be in the range of from 0.001" to 0.003", preferably 0.001" to 0.002". The two alloys mentioned above have been found most satisfactory for use in the present invention, especially Haynes 214.
In the following description, reference will also be made to fibrous ceramic mat, woven ceramic fiber tape, or insulation. Reference may be had to U.S. Pat. Nos. 3,795,524 dated Mar. 5, 1974 to Sowman and 3,916,057 to Hatch dated Oct. 28, 1975 for formulations and manufacture of ceramic fibers and mats useful herein. One such ceramic fiber material is currently available from 3-M under the registered trademark "NEXTEL" 312 Woven Tape and is especially useful herein. Ceramic fiber mat is available as "INTERAM" also from 3-M.
It is a principal object of the present invention to provide a core element which enables simplified, less costly manufacture of a core useful in a catalytic converter.